Cellular communication systems transmit at radio frequencies (RF) that are typically several orders of magnitude higher than the bandwidth of the cellular signal. Additionally, baseband systems for cellular mobile stations and base stations must use a frequency reference in order to generate a modulated baseband signal. Therefore, radio receivers, transmitters, and basebands are implemented using tuned oscillators. In a typical cellular system, on the downlink, tuned oscillators in the base station are used to create a signal and convert it to RF (using a mixer) in the transmitter. A mobile station receives the RF signal, converts it to baseband, and demodulates it using tuned oscillators. A similar approach is followed on the uplink, using tuned oscillators with the roles of base station and mobile station reversed.
The tuned oscillators at the base station and mobile station will each have a frequency error which can cause issues in the cellular system. First, the frequency error between base station and mobile station can either prevent synchronization between them or cause the synchronization to take a long time. Second, the frequency error can cause the demodulation of the cellular signal to fail. One additional problem associated with the frequency error is that it varies with environmental conditions, in particular temperature. Thus, even if the frequency error is small at one point in time, later when the temperature changes the error may also change, causing synchronization and demodulation issues.